This invention relates generally to magnetic resonance imaging (MRI), and more particularly, the invention relates to MRI using steady-state free precession (SSFP) with image artifact reduction.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Magnetic resonance (MR) imaging is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During an MRI experiment, each nuclear spin responds to four different effects: precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MRI experiments, a combination of these effects occurs periodically.
Balanced steady-state free precession (SSFP) sequences have gained popularity in magnetic resonance imaging (MRI) as they can yield high signal-to-noise ratios (SNR) within very short scan times. However, there are several problems limiting the applicability of SSFP imaging. The balanced SSFP signal is a function of the local resonant frequency, leading to characteristic signal nulls/voids (known as banding artifacts) in regions of large resonant frequency variation. Furthermore, the bright lipid signal is often undesired.
At higher field strengths or with longer repetition times (TR), the banding artifacts become more pronounced. It is therefore necessary to limit the off-resonance frequency variation to approximately 2/(3*TR) to avoid any banding artifacts. However, it is not always possible to limit the repetition time as specific absorption rate (SAR) considerations and resolution requirements may place constraints on the minimum TR. A longer minimum TR due to increased power deposition and resonant frequency variations at higher fields can potentially lead to severe banding artifacts.
A common strategy to reduce these artifacts has been to acquire a plurality of SSFP images, where the radio-frequency (RF) pulse phase increment between successive TRs is changed with each acquisition to shift the spectral response of the signal. Several methods for combining these multiple acquisitions have been proposed, including maximum-intensity (MI), complex-sum (CS), and magnitude-sum and sum-of-squares (SOS) combinations. The complex-sum method aims to reduce banding artifacts but is far from optimal in terms of SNR efficiency. On the other hand, the magnitude-sum and sum-of-squares methods yield higher SNR efficiencies but provide less robust suppression of banding artifacts.